1. Field of the Invention
This invention relates to an optical waveguide including a transparent cladding filled with a transparent liquid core having a higher refractive index than the cladding,
2. Prior Art
Solution type optical waveguide tubes comprising a flexible transparent hollow member internally filled with a liquid core are well known in the art. A typical basic structure is shown in FIG. 1 wherein a cylindrical flexible tubing or cladding 1 is internally filled with a liquid core 2 having a higher refractive index than the cladding. The cladding 1 has opposite open ends (only one end shown) in which a window member 3 is fitted for closing the end for containing the liquid core 2 in the cladding 1 in a sealed manner. The window member 3 is a cylindrical transparent solid plug which serves for suitable optical functions of guiding incident or emergent light. The cladding 1 is fastened to the window member 3 by a terminal clamp 4.
One exemplary solution type optical waveguide tube is disclosed in Japanese Patent Publication No. 231904/1987 as a high efficiency optical waveguide comprising a liquid core and a cladding of a polymer having a lower index of refraction than the core liquid. In such high efficiency optical waveguides, fluorinated rubber, fluorocarbon resins, silicone rubber, and butyl rubber are conventionally used as the cladding material, and phosphate oils, fluorinated oils and silicone oils are used as the core material. The material of which the core of optical waveguides is formed should desirably have minimized light absorption in the wavelength spectrum of light to be transmitted, improved light transmittance, and a wide service temperature range, and undergo little interaction with the cladding material (swelling, dissolution, reaction, etc.). However, the phosphate oils conventionally used as the core material, for example, trioctyl phosphate (TOP) have the disadvantages of degradation and yellowing at temperatures above 80.degree. C. interaction with silicone materials used as the cladding even at room temperature, remarkable absorption in the near-infrared region, and poor light transmittance. As compared with the phosphate oils, the fluorinated oligomers are optically superior and heat resistant. The fluorinated oligomers, however, have the risk of interaction with cladding materials, especially fluorinated materials at elevated temperatures. The silicone oils also suffers from the risk of interaction with cladding materials at elevated temperatures.
Therefore it is desired to overcome the above-mentioned problems associated with the core material of optical waveguides.